Add Built-Ins `fgroupsof` and `fwindowsof`

Not sure how to contribute exactly but I just had this cool idea inspired by `fmap`/`map`.
Typically when using `groupsof` or `windowsof` with strings, it's pretty common that you actually want to yield strings and not tuples of chars.
So for example `fgroupsof(3, "ABCDEFGH")` would yield `"ABC"`, `"DEF"`, `"GH"`.

Start by locating the existing `groupsof`, `windowsof`, `fmap`, and `map` built-in implementations and their tests. Confirm the desired string-yielding behavior, including the final shorter group, then add coverage for the examples in the issue.

Written by the indexing model from the issue text.